In the prior art, the generally accepted form of sending an encrypted message is by means of secret key encryption. This prior art system involves the use of a secret key known by the corresponding parties. The secret key is used with an algorithm to transform a plaintext into its encrypted form. As an example: let "sell short" be the plaintext. Let "X" be the key and "the number of the letter in the alphabet/X" be the algorithm. Let the answer be carried two digits to the right of the decimal point. Additionally, the decimal point will not be expressed. In this case let "X" be 5. Thus the first letter of "sell short", is expressed as s=19/X=19/5=3.80=380. "Sell short" would be encrypted as 380-100-240-240-380-160-300-360-400.
In this prior art system, it would superficially seem that the encrypted version and the plaintext version of the message have no similarity. However, an intruder who understood the system could find the key by means of computer analysis and could decrypt the message. Central to this explanation is that the plaintext version of a message is an integral part of all secret key encrypted communications. This makes secret key encrypted messages vulnerable to decryption by a skilled intruder using computers.
Another problem with secret key encryption is the secure exchange of keys between the communicating parties. There must be a face-to-face meeting or some other secure method of key exchange. This can be difficult and/or expensive if the parties are in different geographic areas.
A recent innovation in the field of cryptography is Public Key encryption. This system makes use of inverse functions, that is, a function that is easy to work in one direction and very hard to work in the other direction. Currently, a popular method, (RSA), uses two large prime numbers and their product as a two-part key. While the product can be computed given the primes, factoring the product into the two primes is believed to be beyond present number theory.
Typically the product of the two primes is a number of some 150 digits in length. The aforementioned product is published in a public directory hence the name Public Key encryption. To use the system, one would use the recipient's public key to encrypt a message. The recipient would use one of the primes, (his secret key), to decrypt the message. This method has an advantage over secret key encryption in that it allows communications in a secure manner without the burden of exchanging keys. While RSA encryption is currently thought to be safe, advances in computers and more importantly, in number theory make the prospect of breaking such a system feasible. If an intruder can factor a Public Key into its two primes, he will be able to decipher the encrypted message.